1. Field of the Invention
This invention relates generally to methods and compositions for modifying the permeability of subterranean formations. In particular, this invention relates to methods and compositions for selectively reducing the production of water from subterranean formations by modifying formation permeability using a water control treatment fluid containing a terpolymer in an aqueous-based fluid.
2. Description of Related Art
Production of water and aqueous fluids from oil and gas wells is a common phenomenon which may pose a variety of problems. As hydrocarbons are removed from a producing well, water may begin to move through the formation toward the wellbore where it is produced along with the hydrocarbons. Examples of mechanisms responsible for water production include "coning" into the wellbore from an adjacent water column, and/or movement of water through one or more individual strata completed in the wellbore. Other water production mechanisms, as well as combination of such mechanisms are also possible.
Water production is a phenomenon that typically acts to reduce the amount of oil and/or gas that may be ultimately recovered from a well. In this regard, increasing volumes of water take the place of other fluids that may flow or be lifted from a well, and increase the cost and size of equipment required to separate water from the produced hydrocarbons. Furthermore, operating costs associated with disposal of produced water in an environmentally safe manner typically increase with the volume of produced water, thus increasing the threshold amount of hydrocarbons that must be produced in order to continue economical production of the well. Water production also tends to cause other problems such as scaling of tubulars and surface equipment.
In the past, methods that have been used to control or restrict water production include the gellation of polyvinyl alcohol ("PVA"), polyacrylic acid and the condensation polymerization of phenyl and formaldehyde within formation pore channels. These processes are designed to damage pore channels within the formation matrix and restrict fluid movement through the channels. Another technique involves loading a subterranean area adjacent to the wellbore with a barrier, such as concrete resin or particulate matter, so that the loaded areas are effectively plugged. The area above the loaded area is then perforated to begin production.
In another method, polyacrylamide gels crosslinked with chromium (+3) ions have also been employed. In this case, polyacrylamide may be either a homopolymer or terpolymer. The crosslinking reactions are designed to be slow enough that the fluid may be placed in the reservoir before gellation occurs. The resulting viscous gel is designed to function by blocking or preventing further fluid flow through highly permeable portions of reservoir.
Unfortunately, the above-described conventional water control methods are not selective and may damage both hydrocarbon and water producing pathways. Damage to hydrocarbon-producing pathways is undesirable since it may reduce the well's productivity and profitability.
Other conventional methods have employed high molecular weight polymers in an attempt to achieve selective water control without the necessity of zone isolation methods. Such methods include terpolymers based on polyacrylamide, for example, hydrophillic monomers such as ammonium or alkali metal salts of acrylamidomethylpropane sulfonic acid ("AMPS"). In many cases, these polymers have provided poor performance due to inadequate retention within the formation. Addition of anchoring groups, such as dimethyldiallylammonium chloride, have been used to achieve some improvement in performance, but the resulting polymers may still not be adequately retained.
A further disadvantage to many conventional water control compounds is that they are unstable in acids and heavy brines which are frequently found within oil and gas wells, and/or they may degrade with increased temperature. These characteristics may render the compounds useless in many downhole applications.